static inline unsigned long get_current(void)
{
    unsigned long current;

    asm volatile (
    " movl %%esp, %%eax;"
    " andl %1, %%eax;"
    " movl (%%eax), %0;"
    : "=r" (current)
    : "i" (0xfffff000)
    );
    return current;
}

program.c: Assembler messages: program.c:455: Error: incorrect
register `%rbx' used with `l' suffix

asm volatile (
" movq %1, %%rax
" andq %%rsp, %%rax;"         // We only care about lower bits.
" movq (%%rax), %0;"
: "=r" (current)
: "i" (0xfffffffffffff000)
: "rax"    // Remember to mark any temporary registers modified in "clobber"
);

static inline unsigned long get_current(void)
{
    unsigned register long current asm("sp");

    asm volatile ("" : "=r"(current));

    return (current & ~ (unsigned long)0xfff);
}

sp

sp